Stereolithography is a relatively new technology which permits the direct translation of a graphically-generated object into a solid, 3-dimensional object. Many aspects of stereolithography are described by C. Hull in U.S. Pat. Nos. 4,575,330 and 4,929,402. This technology is very useful in prototyping applications, because the prototypes can be rapidly and accurately formed from computer-generated graphic models.
Briefly summarized, one popular version of stereolithography involves the generation of a 3-dimensional object on a computer, using computer-aided design and engineering software. The computer is then, in effect, instructed to mathematically divide the graphical object into thin sections or "slices". Each section can then be treated as a two-dimensional object having x, y coordinates. The computer is then instructed to direct the position of a laser beam according to the dimensions of those coordinates. The laser is directed onto the surface of a container of a liquid--usually some sort of photopolymerizable material. Polymerization occurs only in those areas which are contacted by the laser beam, thereby forming a solid polymer. As each two-dimensional section is formed, a movable platform lowers the layer to a position just below the surface of the polymerizable liquid. Then, the next "slice" is drawn on top of the previous slice by the action of a laser. The process is repeated, stacking polymerized layer upon layer, until the entire solid object is formed.
Clearly, stereolithography is a very attractive technology in the design field. Complex models can be rapidly and automatically produced by such a technique, in contrast to older processes which required great amounts of time and effort on the part of trained craftsmen in achieving the precise dimensions required. The prototypes prepared by stereolithography are used in the design of articles made from a wide variety of materials, e.g., metal, glass, plastic, and wood. The end products are countless: automobiles, airplanes, turbine engines, windmills, boats, electrical connectors, machine tools, electronic devices, etc.
It's clear that stereolithography depends to a large extent on the chemical process used to polymerize the liquid monomer. Thus, the capabilities of stereolithography are limited to some degree by the related polymerization chemistry--most often, photopolymerization. As an example, the final, solid object must be dimensionally faithful to the computer-generated design. Current stereolithographic applications require the formation of mechanically stable photopolymers that are true to within 0.005 inch of design. Such an exact tolerance requires precise contact of the laser beam with the specified dimension of the monomer mixture.
Typical materials currently used in photopolymerization for stereolithography include acrylate or vinyl ether-based photopolymers. However, these materials sometimes shrink during photopolymerization. For example, acrylate photopolymers may shrink about 6-8% in volume, resulting in considerable distortion in the shape of the final object.
Furthermore, the presently-used photopolymers often do not have a glass transition temperature (T.sub.g) high enough to prevent warpage on standing, or on exposure to moderate temperatures present during subsequent processing steps, e.g., drilling, machining, or sanding.
Moreover, some of the acrylate photopolymer resins tend to be toxic and skin-sensitizing. Workers who become sensitized to the photopolymers must avoid all future contact with the resins. Special precautions must be taken in the placement of stereolithographic equipment to avoid undue worker exposure, and to comply with safety and health regulations.
In view of some of the drawbacks associated with currently-used photopolymers for stereolithography, a search has been in progress to replace such materials with alternative systems. However, the search has been largely unsuccessful and unsatisfactory for various reasons. For example, some of the proposed replacement systems exhibit poor photosensitivity to laser-produced UV light. The photosensitivity (or "photoresponse") of a photopolymer determines the speed with which the image can be "written onto" the surface of the polymerizable liquid. Thus, poor photosensitivity will greatly decrease the speed at which the solid, 3-dimensional object can be generated, consequently decreasing the efficiency of the overall process.
It's thus apparent that the art of stereolithography still requires photopolymerizable materials which exhibit good photosensitivity, and which minimize any distortion of the polymerized object, leading to a very precise reproduction of the imaged object. Furthermore, these materials should be relatively innocuous from a health and safety standpoint.